The present invention relates to an image reader applicable to an image forming apparatus for generating a digital video signal representative of an image printed on a document by reading the document. More particularly, the present invention is concerned with an image reader capable of erasing an image associated with the background of a document while reliably preventing an image printed on the document from being omitted.
An eletrophotographic copier, facsimile apparatus, laser printer or similar image forming apparatus has an image reader, or image scanner, which is usually implemented by a one-dimensional image sensor. While main scanning is effected by the iamge sensor which scans a document, subscanning is effected by the relative mechanical scanning of the image sensor and the document. By the main and subscanning, the distribution of image densities on a document is sequentially read. The image densities are delivered in the form of an analog signal, while an analog-to-digial (AD) converter transforms the analog signal into a digital signal. A prerequisite with this kind of apparatus, such as an electrophotographic copier, is that unnecessary images existing on a document be prevented from being reproduced while necessary images are reliably reproduced, i.e., an image associated with the background of a document which is not necessary be erased. To meet this requirement, is has been customary to regard output images, whose densities are lower than a predetermined level, as unnecessary images associated with the background and to convert such images to a zero density level.
The above-described prior art implementation is not satisfactory because the density of a necessary image and that of an unnecessary image vary over a broad range depending upon the kind of a document. For example, a diazo copy has a background having an extremely high density. Concerning a relatively thin document, an image printed on the back is read through the document to constitute an unnecessary image in the background, increasing the density of the background. Conversely, a low-contrast document or a document on which information is additionally written with a pencil has a necessary image which has a relatively low density. Such an image is often treated as an unnecessary image and therefoe lost on a reproduction. When a document is read by ordinary optics, a reflection from the document is reflected again by a light source to contribute to imagewise exposure. Hence, the actual amount of exposure depends upon the reflectance of a document, i.e. the kind of an image printed on a document. For example, the amount of exposure difers by about 10% from a document the entire surface of which is white to a document the entire surface of which is black. As the amount of exposure is changed, the density of an image read by the optics is also changed resulting in the density of an unnecessary image associated with the background being changed in a signal processing system with respect to the signal level. This causes the degree to which the unnecessary image is erased to fluctuate. Specifically, an unnecessary image which may appear in the background after a solid black image which is necessary is sometimes left non-erased. In order to accommodate for changes in the density of a document and thereby to erase an unnecessary image while preventing a necessary image from being lost, a control is possible wherein the densities of a document image are detected to determine a signal level representative of an unnecessary image which exists in the background, and the determined signal level is caused to follow a level which has been determined to be a background signal level by signal processing.
For such a control, use is made of a level detecting circuit which detects the peak level of a video signal which is representative of the background of a document. With this kind of level detecting circuit, it is necessary to increase the negative-going signal level associated with an output image so that the signal level associated with an unnecessary image may be prevented from following that of a necessary image. If follows that upon a transition from a light unnecessary image to a dark unnecessary image both of which exist in the background of a document, the signal level associated with the unnecessary images cannot be detected without a substantial delay. When a document is reproduced by a diazo process, for example, misalignment of the document and a paper sheet causes a leading edge portion of the paper sheet or copy to appear white and the other portion to appear blue. That is, the background density sharply changes on the same copy. When this kind of paper sheet is used as a document to be read, the signal level associated with the background is sharply lowered in the leading edge portion. Then, the level detecting circuit fails to follow the signal level with the result that background is not erased over an image portion of several centimeters which follows the leading edge portion. With a diazo copy, it often occurs that the background density noticeably changes due to irregular printing. Also, the background density of a document undergoes a sharp change when the document is prepared by adhering some cut pieces of paper each having a substantial density to a single paper. With a digital image reader, it is preferable that the background level of a document be determined before AD conversion so as to remove a signal level associated with unnecessary images from a digital image signal. This is successful in minimizing the required number of bits of an AD converter and therefore in simplifying the construction of electrical circuitry.